单项选择题

With what topic is the second paragraph mainly concerned

A．How the Sun evolved.
B．The structure of the Sun.
C．Why scientists study the Sun.
D．The distance of the Sun from the planets.
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单项选择题
All of the following are parts of the Sun's atmosphere EXCEPT the ______.
A．corona
B．chromospheres
C．photosphere
D．core
单项选择题
According to the passage, as the corona rays reach the planets, they become ______.
A．hotter
B．clearer
C．thinner
D．stronger
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